Question

when is the velocity of a body said to be constant

Answer 1

when the velocity of a body is not uniform, the motion is accelerated. A body moving with a constant velocity in an accelerated motion. When a body is moving with a constant velocity, then its acceleration will be zero, because then the change in velocity (v - u) is zero.

Answer 2

To have a constant velocity, an object must have a constant speed in a constant direction. Constant direction constrains the object to motion in a straight path thus, a constant velocity means motion in a straight line at a constant speed.